Latest turbine experience clearly shows that mechanical properties of coatings are one of the most critical material issues in advanced turbines. Progress in blading materials and technologies (i.e. single crystal blading) is not followed by today coating systems. Thermo-Mechanical-Fatigue (TMF) properties of coated alloys are far below those for uncoated single crystal (SX) material. The main effort in increasing of the TMF life of the coated component concentrates now in two directions. The first is to fit the coating composition to the composition of the substrate. That minimizes the difference in thermal expansion between coating and substrate. The second is in providing a fine grain structure to the coating in order to increase ductility and therefore reduce stress accumulation due to the difference in mechanical behavior of the coating and the substrate.
U.S. Pat. No. 4,758,480 discloses a class of coatings whose composition is based on the composition of the underlying substrate. The similarity in phase structure and in the chemical composition renders the mechanical properties of the coating similar to those of the substrate thereby reducing thermomechanically-reduced damage during service. However, when this coating is applied by traditional means on the single crystal substrate, the difference in the E-modulus between &lt;010&gt; oriented surface layer of the substrate and randomly oriented coating grains produce high TMF damage.
U.S. Pat. No. 5,232,789 discloses the further improvement of the TMF properties of the coating-substrate system. The coating, which has composition and phase structure similar to the substrate alloy, has at least 1000 times more fine-grained structure, produced by a special technology. The lowermost interface portion of the fine-grained coating grows epitaxially, and therefore has the same crystal orientation as the substrate. Epitaxial growth solves also coating/substrate interface adhesion problem.
However, the system of a single crystal substrate and the multicrystal coating still has a large difference in the mechanical behavior between the substrate and the coating as any equiaxed structure possesses E-modulus much higher than those for single crystal material in &lt;001&gt; direction. Higher E-modulus reflects in lower TMF life of the coating compared to the substrate (although the stresses on sub-strate-coating interface are significantly reduced compared to the traditional coating-substrate system). Multiple grain boundaries drastically reduce the creep resistance of the fine-grain coating, which finally determines life of the entire blading system.